High power linear RF amplifiers are known devices that have heretofore been used in signaling systems requiring broad bandwidth operation. Such amplifiers find application in many fields, including the cellular telephone communications systems which employ ultra high frequency RF for communications transmissions.
In such systems, the linear amplifier processes a number of different communications channels simultaneously. For example, a current band of UHF frequency operation used in multi-cellular telephone systems is 870 through 890 MHz. That frequency range or band is divided into many separate bands or "channels", each of which carries the carrier and modulation information associated with a single cellular phone user, as assigned by the telephone station equipment at the time the user places a call on the cellular phone. Within the described UHF band, typically, at least 16 or more separate channels are provided. Because of its low distortion characteristics, the linear amplifier is capable of amplifying simultaneously signals from all of the 16 or more channels without allowing telephone modulation signals from one channel to cause interference with the telephone modulation signals of another channel.
The prior linear amplifier assembly includes a plurality of semiconductor amplifiers, or amplifier modules as they are sometimes called, a power splitter and a power combiner. In such known assembly, the RF signal or signals to be amplified and which may consist of a plurality of separate Rf signals spaced apart in carrier frequency or channels, as previously described, is applied to the power splitter input. The power splitter divides the signal between the inputs of the plurality of semiconductor amplifier modules and a portion of the original input signal is amplified by each of the semiconductor amplifiers. The amplified signal from each amplifier is then applied as an output to a corresponding one of the multiple inputs of the power combiner. The power combiner in turn adds together or combines the individual RF signals inputted from the semiconductor amplifiers into a single output signal of a high power level; one that is amplified in level from the signal level applied to the input at the splitter and providing a RF power gain. The combiner output is then transmitted to other system equipment, not relevant to the present subject.
The requirement for multiple numbers of semiconductor amplifier modules in such linear amplifier assemblies is due to technological limitations existing in present day semiconductor amplifiers. No single known reasonably priced semiconductor amplifier is capable of operating at the high power levels and frequencies required in such communications systems; typically power levels of 300 watts average and over 1 kilowatt peak power in frequency ranges of between 870 and 896 MHz. This limitation is even more pronounced at frequencies beyond the UHF range and extending in the microwave frequency regions, those above 1 GHz, all of which may be employed with linear amplifiers as described.
Semiconductor amplifier modules for such systems are available from many sources including those offered as model ABC-900-60 by the RF Devices Division, TRW, Inc. of Lawndale, Calif., U.S.A. Those semiconductor amplifier modules find application in the present invention as well as in the prior art linear amplifier assemblies as becomes more apparent in subsequent portions of this specification.
In the prior art linear amplifier assembly described, reference was made to a power splitter and a power combiner. As those skilled in the art appreciate, these are essentially the same devices; the device is more typically referred to as a combiner-splitter. The same component is used in one mode to divide an input signal into a plurality of output signals and is referred to by its "splitting" mode of operation. Conversely, when multiple input signals are applied to those multiple terminals, previously referred to as the splitter output, the signals are additively combined at the output, previously referred to as the splitter input, and, hence, the component functions as a "combiner".
In the linear amplifier combination, the RF characteristics of the splitter and the combiner must be essentially the same, particularly, bandwidth, phase delay and impedance characteristics as well as other electrical characteristics as is known to those skilled in this art. To insure such compatability of those RF characteristics, the prior designs used the easiest, most reliable and cost effective approach: they used the same combiner-splitter structure in both instances; one splitter-combiner functioning in the system as the splitter and the other splitter-combiner functioning in the system as the combiner. As becomes apparent from the description hereinafter, the present invention departs from that practice in achieving a novel and effective assembly.
One particularly useful form of splitter-combiner used in a linear amplifier system is a radial splitter-combiner, and, particularly, a radial wave guide splitter-combiner. That known component is of a geometry of a circular disk in which there is centrally located on one side coaxially of the disk an electrical connector, such as a coax connector, and further includes a plurality of individual secondary connectors, the exact number being dependent on the number needed in the particular system, sixteen in the specific system example given, evenly angularly spaced at a given radial distance from the center of the disk. The secondary coaxial connectors may be located on one side or the other side of the disk as the designer's choice. Within the metal walls defining the disk like shape, there is included a chamber and a series of ridges and grooves or channels of ringlike circular shape is formed concentrically about the central axis of the element. As is known to those skilled in the art, the width and height of the ridges and the depth of the channels as well as the spacing between the edges of the ridge and the opposed wall surfaces within the internal chamber serve to define the electrical transmission characteristics of the combiner-splitter. An electrical probe extending from the center coaxial connector is electromagnetically coupled to the within chamber as are the conductors associated with each of the plurality of the perphierally located secondary connectors.
When used as a splitter in such systems, the RF signal applied to the center conductor is radially propagated internally to all of the outer located secondary connectors and is, hence, divided between them so that a portion of the input signal appears at each of the output connectors. Conversely when used as a combiner, the RF signals from a plurality of sources is applied to respective ones of the outer secondary connectors and each signal radially propagates within the chamber inwardly toward the center connector. The arrangement is such that the multiple signals arrive at the center probe location in phase so that the separate signals are added or combined. The RF signal outputted from the center connector is thus a combined or higher power signal.
Although the prior linear RF amplifier assemblies constructed according to the conventional practice performs the function very well and is electrically very efficient, it is nonetheless mechanically cumbersome and requires many expensive components, such as cables and connectors. For example, in a system that required the use of 16 semiconductor amplifier modules, a radial wave combiner and a radial wave splitter, there were approximately 32 pieces of coaxial cable and approximately 128 coaxial connectors. Further each of the two splitter-combiners require a total of two circular metal plates, at least one of which is machined to close tolerances in order to attain the necessary structure. The bulky and complex mechanical arrangement is exemplified by the depiction appearing in an advertising brochure handed out by MMD Company in which a linear RF amplifier assembly made by the Microwave Modules and Devices, Inc. Company is illustrated. More cumbersome yet is the RF linear amplifier assembly produced prior to the invention by TRW, Inc. at its Bordeaux, France facilities. The present invention reduces the number of expensive parts, including the elimination of coaxial cables of extended length and associated connectors, and reduces the number of expensive machined parts required in such assembly.
In normal operation, semiconductor amplifiers generate heat as an undesirable side effect of the amplifiers amplification function. If the temperature at which the semiconductor operates becomes elevated, the life of the semiconductor is shortened. If that temperature rises to a level well beyond the specification or limits of the semiconductor, the heat destroys it entirely. Thus, the semiconductor amplifiers include heat sinks and heat fins, extended metal vanes, typically of aluminum, through which heat generated in the semiconductor material during operation is conducted externally to the ambient assisting to maintain the operational temperature of the semiconductor within specification. A further element thus included in conventional linear amplifier systems as well as in the present invention is a cooling system, such as a fan or the like which circulates air, for cooling the heat sensitive electronic components. In conventional systems, the cooling arrangement adds to the cumbersomeness and mechanical complexity. Typically the system includes a fan for drawing air across the heat fins and exhausting that air in a different direction; the movement of air serving to more rapidly and effectively transfer away or dissipate the generated heat.